Vacuum electric tube



Patented; Mar. 22, 1932 1? BATE mm; g

nnunsr ROBINSON, or wrrmueronynuemun I *vAoUuM' merald was I i i no Drawing. Application filed May 24,i1924,'Seria1No. 715553, and in 'GreatBritain irune'lz, 192s;-

Thisinvention relates to vacuum electric tubes and partic'ula-rly to electron emitting cathodes of, the oxide coated "type. These cathodes of say platinum or Iplatinum-iridb "5 um are usuallyco'ated withoxides of calcium, barium? or strontium for 'exa1nple,'or a; mixture of such oxldes, In practice a com-' mon method is toapply acoating of a compound of the metal, usually the nitrate, with an organic binder, by paintingor dipping the cathode; which binder is subsequently re moved byheat-ing in an oxidizing atmosphere and the compound decomposed to the.,ox1de.

c According to this invention an I electronemittingycathode of the typeabove referred to is produced in the follovvin'gynuanner. The

c cathode isfirstcoated'with the metal, or a mixture of the ymetals, otthe compounds which form the electron-emitting coatings, and; the *metal surface is then converted to the compound desiredby suitable means. For instance, a cathodevconsisting of a sheet,

wire or-filament ofrsuitable material may be;

7 coated-With an alkaline earth metal, such'has 'calcium,barium or strontium, or a mixture of these metals in any desired proportion and the surface then oxidized by-contactfwith,or heating in, airjjor oxygen. The cathode-base or support-for the coating may consist IEOIQX'. ample of tungsten,- molybdenum, platinum, platinum-iridium alloy, orcarbon'. The coat: ing metalmay be applied to the supportby electro-deposition, .byprecipitatioh :or. de-

composes into the" metal on heating, or by deposition by means of cathode sputter-mg in; vacuum. In'the latter'c'ase the alkaline earth coating metal maybe volatilized on to,

The metalis'then converted to oxide heats:

position'from a compound which readily odesupport or base as the cathode for-electrolysis;'

ing in air, or alternatively it may be totally vor'partially oxidized while it is still immersed with thes'upport in theacetone solution, the support or base being heated subsequentlyin air; a

Th" invention: will be' particularly de scribed byway of example, in its applica tion' to coating a cathode of platinum or molybdeniun with the oxides of alkaline earth metals. 'Ashereinbefo'restated, preferably 0 the alkalineearth metal is 'depositedfonthe filament by electrolysis of an acetone solutionof the iodide To prepare the electroly tic solution in one method a quantity of one of the alkalineearth metal iodid'esis "placed in' a distilling flask and. heated in a current of hydrogen to atemperature 'between 18O C, i and 200 (lg'for a period of at least two hours,- in order to secure complete dehydration.

The dehydratediiodide'is then mixed'with pure acetone and thesolution is made up with acetoneftoabout half normal strength; 1 In another-method of preparing the electrolytic bath-, th'eiodide of an alkaline earth metal isfdissolved in acetone'and the resulting solu- -7 l I tion is'dr ed over anhydrous sodium sulphate or calcium chloride for two orfthree days. 1 I The cathode of platinum is first cleaned by placing it in concentrated nitric acidfollowed byammonia or by boiling in caustic sodasolu 8 tion, with subsequent washing in water. 'If tungsten or molybdenumis employed for the cathode, then it is cleaned in the case of Wire by heating in hydrogen; or in'the caseof sheet or tube by dipping it into molten'sodiumor V potassium nitrate, which is finally caretully removed from the cathode by washing; The cathode is then'lowered into the electrolytic depositing bath; It is obvious that a :number i of cathodes'may be coated at the same time,

and thatfthey may be in any suitable form, such for example as hairpin loops. The

metal is electrolytically deposited on thefilament to the required. thickness, and forjthis purpose they voltage efiecting'depositionis advantageously continuously raised from a low to a high value during deposition; some timesitis found-that volts are' fi'nally q necessaryto keep thede'position current fairly i constant The filament isf advanta'geously 9 I M50309 a coated with a mixture of alkaline earth metals rather than with a single alkaline earth metal. This may be effected by two electrolytic deposition processes employing solutions of strontium iodide and barium iodide. The filament is placed alternately in each electrolytic bath thus coating it with alternate layers of. strontium and barium; or it may be coated with two layers only. The metals deposited alloy with each other when the filament is subsequently heated. Or a mixture of barium and strontium may be deposited from a single electrolytic bath containing a solution of mixed barium and strontium iodides.

After coating,the filament with the desired alkaline earth metal is removed from the bath and the metal coat isconverted to oxide. This may be efi'ectedin various ways. For example it is sufficient to leave the filament in dry air, butit is preferable to heat it to a temperature between 600 C. and 700 C. in air for half-an-hour to two hours. By this means the coated metals are alloyed with each other,converted to oxides and the oxide coating firmly bonded to the filament.

The oxide coat produced by any of the above described processes may then be converted if desired to a carbonate coat.

filamentwithacarbonate coating can be han- I refractory metal, such as molybdenum or dled with ease, and can be left in a moist atmosphere without injury, before its subsequent assembly in a valve, whereas an oxide coatedfilament often requires a further protective coating. The carbonate coating 1s produced by heating theoxide coated filament to a dull red heat in carbon dioxide and permitting it to cool in an atmosphere of the same gas. In this method of preparation the filament is heated to between 600 C. and 700 C. in air, for about minutes, after removal from the depositing bath, and is then heated in a stream of carbon dioxide for about minutes, the filament being left cold in the carbon dioxide for a further 30 minutes. On assembly within the valve the carbonate is reconverted to oxide by heating.

The coat may be oxidized while it is still in the depositing bath by permitting a little water tobe present. The coatin is deposited as metal and is then oxidized y the water present in the solution, the cathode subsequently being heated. The exact action of the water is not known, but it appears that a complex oxygen compound is formed probe ablycombined with acetone. Although this methodof roduction is not preferred it may sometimes e employed for coating special cathodes.

When the supporting base consists of a material whichis capable of alloying with the alkaline earth metalsfat a temperature below the volatilizing point of said metals, such for example as platinum, then the alkaline earth metal may. .betialloyed thereto by beating out of the presence of oxidizing gases before the oxidizing process is effected. For instance, a platinum filament coated with a mixture of barium and strontiummetal may be heated for a few moments in pure hydrogen, whereby subsequently a more homogeneous and amore firmly bonded coat is obtained. It will berealized, however, that this alloying process cannot be carried out when metals such as molybdenum or tungsten form the supporting base. 1

Although t is better to heat the filament in an inert or non-oxidizing atmosphere when itis desired to alloy it to the supporting base, it is not necessary for if the base is clean and the metal coat is not very porous,small but appreciable alloying will occur if the filament is flashed in air. Preferably in this case the filament is flashed for a moment or two to a temperature of say 1100C.to 1200 C. after which period it is baked in the usual manner.

For some purposes it is desirable however that the barium and strontium metal should not be alloyed to the base, since often the subsequently produced oxides are found to have eaten in to the base, thus making it brittle. If the base is in the form of. a thin filament, the uniformity of its resistance is thus stroyed.

If the supporting base consists of a strong tungstenthen after being coated it is preferably not heated in air, since oxidation of the molybdenum or tungsten takes place and the coating falls off. In this case, the coated tungsten or molybdenum is heated for about ten minutes at'a dull red heat in hydrogen of commercial purity. -There is sufiicient oxygen or oxygen containing material such as water present in the hydrogen and in the metal support to effect partial or complete oxidation of the alkaline earth metal de posits. The coatingmay then be converted to carbonate ifdesired in the manner described above.

By the use of this invention cathodes of tungsten and molybdenum may be coated with an adherent coating of alkalineearth oxides or a mixture of these; and it is particularly applicable to cases in which thecathode is not filamentary such for example, as when it consists of a sheet or tube of tungsten or molybdenum. i

g It is understood that the invention may be applied generally to the. coating of cathodes with compounds or mixtures of compounds of any metalfor obtaining a desired electron emission, and is not limited to the processes described by way'of example.

I claim as my invention 2- y 1. The method of making electron-emit ting cathodes'for vacuum electric tubes by coating a suitable support with at least one metal, converting said coating into a comwhich has desired electron-emitting properties, and then convertingsaidcoating j to a carbonateby h'eating in an atmosphere 1 of carbon dioxide, whereby the cathode can readily be handled before assembly.-

said coating.

3. The method of making an electron-emit- I ting cathode of the oxide-coated type, which consists in: depositing on a metal cathode a coatingof at least one alkaline earth metal and substantially said coating. y I y 4. The method of making an electron-emitting cathode of the oxide-coated type, which consists .in electrolytically depositing on a suitable support a coating of at least one alkaline earth metal from .a solution thereof i in acetone containing a trace of water.

5. The method of making an electromemit- I tingcathode of the oxide-coated type, which I said coating While out 0t contact with'atmosconsists in producingjo'n a metal cathode a coatingof at least one alkaline earth metal" and substantially simultaneously oxidizing pheric air.

' In testimony whereof I have hereunto subscribed my namethisninth day of Ma ,1924.

ERNEST YEOMANROBIN ON.

simultaneously oxidizing, V 

